The present invention relates generally to a flow rate measuring device for detecting a flow rate of a fluid. More particularly, the invention relates to a thermal type flow rate measuring device which defines an auxiliary passage within a main passage flowing a fluid and measuring a flow rate of the fluid flowing through the auxiliary passage.
As prior art of a thermal type flow rate measuring device, there is a thermal type flow meter disclosed in Japanese Patent Application Laid-Open No. Heisei 9(1997)-304140. The disclosed thermal type flow meter is formed with an L-shaped auxiliary passage in a flow passage, and a sensor element for detecting flow rate is arranged within the auxiliary passage.
On the other hand, an air flow meter disclosed in Japanese Patent Application Laid-Open No. Heisei 9(1997)-287991 includes a reversed U-shaped auxiliary passage formed within an air passage. A sensor element for detecting flow rate is arranged in the vicinity of the outlet portion of the auxiliary passage.
In case of the air flow rate measuring device for measuring an intake air of an automotive vehicle, while traveling behind a vehicle splashing a large amount of water under rainy condition, while traveling in heavy rain, rain water may easily pass through an air cleaner to penetrate into an air intake passage in a form of fine mist.
It has been known to artisan in the art that the thermal type air flow rate measuring device inherently cause output error as adhering water droplet on the flow rate detecting sensor element and keeps output error until the adhered water droplet is evaporated completely. When error is output in out put of the air flow rate detecting sensor for automotive vehicle, it becomes difficult to obtain appropriate air/fuel ratio to cause difficulty in maintaining normal engine revolution.
In case of the technology disclosed in the foregoing Japanese Patent Application Laid-Open No. Heisei 9-304140, when moisture penetrates into the air intake passage, it may directly collide on the flow rate detecting sensor element to adhere thereon to cause difficulty in obtaining accurate output.
On the other hand, in case of the technology disclosed in Japanese Patent Application Laid-Open No. Heisei 9-287991, since the flow rate detecting sensor element is disposed in the reversed U-shaped passage. Therefore, possibility that the water droplet directly contact with the flow rate detecting sensor element to adhere thereon, is quite low.
However, even with the U-shaped auxiliary passage structure, it is not possible to completely avoid adhesion of water droplet on the sensor element. Namely, when moisture penetrates into the auxiliary passage, a part of mixture may adhere on the peripheral wall of the auxiliary passage initially in a form of small water droplet. Then, small water droplets are coupled to form large water droplet.
Since large water droplet may be easily brown by air flow, the water droplet thus blown may adhere on the sensor element.